1. Field of the Invention
The present invention relates to an image sensor including a microlens array and a method of manufacturing the same, and more particularly, to an image sensor including a microlens array and a method of manufacturing the same, by which a lens may be manufactured using a semiconductor material. The method enables a lens to be monolithic or hybrid integrated with an image sensor, which makes the image sensor have a high numerical aperture (NA), a subminiature, high-density, polygonal microlens array.
2. Discussion of Related Art
Generally, the improvement of signal-to-noise ratios (SNR), resolution, and sensitivity has become an important issue in developments of image sensors, particularly, high-performance infrared (IR) image sensors.
A main cause of a noise of an IR image sensor may be a dark current. Since the dark current is linearly proportional to the area of the photodetector, a photodetection region should be reduced to improve an SNR.
In addition, since the number of pixels per unit area should be increased to improve the resolution of an image sensor, the area of each pixel including a photodetection region should be reduced.
However, a reduction in the photodetection region may lead to a reduction in the amount of light received by a photodetector, which may bring about a reduction in the light reception efficiency of the photodetector, thus resulting in a drop in the sensitivity of the image sensor.
Thus, a microlens array including a plurality of microlenses may be formed on an image sensor. The microlens array, which may focus light incident to a pixel on a small photodetection region of the pixel, may function to effectively increase the amount of light incident to a photodetector even if the photodetection region is reduced.
Accordingly, many methods of manufacturing a microlens array integrated in an image sensor have been proposed thus far. A representative method of manufacturing an image sensor including a microlens array may include reflowing a polymer.
A method of reflowing polymer may include forming a transparent predeposition layer on a substrate including an image sensor, forming photoresist patterns for a plurality of microlenses on the transparent predeposition layer, and reflowing the photoresist patterns by heating. As a result, since the polymer tends to have a globular surface due to surface tension, a microlens array having a predetermined radius of curvature may be formed.
However, each of the microlenses formed using the above-described reflow method may have a naturally circular shape, and thus the fill factor of the microlens array (i.e., a ratio of the area of the microlens to that of a square unit pixel region or unit cell region of the microlens array) may be limited to 78% or lower.
That is, at least 22% of light incident to each of square pixels may be incident to a region where no microlens is formed. Accordingly, at least 22% of the light incident to each of the pixels may not be focused on the photodetection region but be lost without contributing to forming images.
Furthermore, since a method of manufacturing a microlens array using the reflow of a polymer was performed using surface tension, it was difficult to control the radius of curvature or focal distance of the microlens, which doesn't allow a high-density of microlens array.